Just judging by Andrew House's comments to date, I don't think they see rolling generations happening like this. They still seem to believe in the model of a fixed software generation, within which we might have a couple of hardware iterations.
That being the case, Neo won't be a PS4 that turns into a PS5, dropping the first PS4 out of the equation. It'll always just be a PS4. The next compatibility step will be the first PS5 model, whenever that comes.
All that said, I think Sony (and Microsoft) are still figuring out what might happen in the future, so maybe that could change. But right now it doesn't sound like it's on the cards, that the software cycle will hop along to straddle a window of hardware models at a given time. If they see Neo like that - as a PS4 iteration rather than a potential PS5 entry model - then I think it and the first PS4 will always play the same software.